This invention relates to automatic frequency control for radio receivers, and more particularly relates to such control employing frequency error detection.
Regulatory agencies have recently imposed a tighter frequency packing of channels (Mode 0 Narrowband) for the next generation VHF commercial air traffic control links. The new regulations have created a need for automatic frequency centering of the received band pass signal in order to prevent filter distortion and to improve receiver sensitivity performance. The new regulations also necessitate techniques for dealing with the inherent frequency drift of transmitters and receivers as well as the frequency shift due to doppler. This invention addresses all of these problems and provides a solution.
The preferred embodiment is suitable for use in a radio receiver of a carrier signal including an error which shifts the frequency of the carrier signal relative to a first reference frequency. The carrier signal is modulated by a baseband signal containing information to be detected. The information is required to lie within an allowed frequency range.
The preferred embodiment provides automatic frequency control by generating a residual carrier signal modulated by the baseband signal in response to the carrier signal. The generation of the residual carrier signal is preferably accomplished by a converter. The residual carrier signal has a residual frequency range within the allowed frequency range. The frequency of the residual carrier signal is shifted to generate a frequency shifted signal in response to a control signal. The frequency shifting is preferably accomplished by a frequency shifter. The control signal is generated in response to any frequency error in the frequency shifted signal relative to a second reference frequency. The control signal preferably is generated by a frequency detector. As a result of the generation of the control signal, the frequency error in the frequency shifted signal is reduced. Information from the frequency shifted signal is detected, preferably by an information detector. The frequency shifted signal is filtered, preferably by a filter which passes frequencies within the allowed frequency range.
By using the foregoing techniques, frequency can be controlled to reduce cross talk and to provide improved radio reception within a relatively narrow band of allowed frequencies with a degree of accuracy and economy previously unavailable.